1. Field of the Invention
The present invention relates to an X-ray image intensifier for converting an X-ray image into a visible optical image or electrical image signal and a manufacturing method of the same and, more particularly, to an improvement in the brazing structure of an X-ray incident window of an X-ray image intensifier and an improvement in a method of brazing the X-ray incident window of an X-ray image intensifier.
2. Description of the Related Art
An X-ray image intensifier is useful for examining the internal structure of a human body or object, and is used for converting the transmission density distribution of X-rays irradiated on the human body or object, or an X-ray image, into a visible optical image or electrical image signal.
What is required in an X-ray image intensifier is to convert the contrast or resolution of an X-ray image into a visible optical image or electrical image signal faithfully and efficiently. In practice, this faithfulness is influenced by the respective constituent elements in the X-ray image intensifier. In particular, since the conversion characteristics of an X-ray input section are inferior to those of an output section, the faithfulness of the output image is largely influenced by the characteristics of the input section. In an input section which has been conventionally used practically, a thin aluminum substrate is placed inside the X-ray incident window of a vacuum vessel, and a phosphor layer and a photo-electrical cathode layer which serve as an input screen are adhered to the rear surface of the substrate. With this structure of the input section, since the total incident X-ray transmittance is low and the X-rays scatter largely, a sufficiently high contract and resolution are difficult to obtain.
A structure in which an input screen consisting of a phosphor layer and a photo-electrical cathode layer is directly formed on the rear surface of an X-ray incident window serving as part of a vacuum vessel is described in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 56-45556 and European Pat. Appln. KOKAI Publication No. 540391A1, and is thus conventionally known. In this structure, since the X-rays are transmitted through only the X-ray incident window of the vacuum vessel, a decrease in transmittance of the incident X-rays and scattering of the X-rays can be increased, so that a comparatively high contrast and resolution can be obtained.
The input screen consisting of the phosphor layer and the photo-electrical cathode layer is formed to have an optimum curved surface to minimize a distortion in image on the output screen caused by an electron lens system. For this purpose, the input screen is often formed to have a parabolic surface or a hyperboloid in place of a surface having a single radius of curvature.
Although the structure in which an input screen consisting of a phosphor layer and a photo-electrical cathode layer is directly formed on the rear surface of the X-ray incident window of a vacuum vessel is already widely known as a technique, it has not reached a sufficiently practical level yet. The major reason for this is that since the X-ray incident window of the vacuum vessel is deformed by an atmospheric pressure, the input screen is not stably adhered to the rear surface of the X-ray incident window, and an image distortion can be easily caused by an electron lens system. In an ordinary X-ray image intensifier, even if an electron lens system including an input screen is designed to have an optimum size and shape, if the input screen is deformed to be partially moved to the vacuum or outer air side by as small as, e.g., 0.5 mm, a satisfactory output image cannot be obtained due to a distortion in the electron lens system.
An input screen, in particular a phosphor layer excited with the X-rays, is formed by vacuum deposition to have a comparatively thick fine columnar crystal structure, so that it can obtain a high resolution and a high X-ray detection efficiency. In a method in which vacuum deposition is performed by placing an X-ray incident window in a deposition apparatus, however, the crystal structure of the obtained phosphor layer is largely influenced by the substrate temperature of the X-ray incident window. For example, since a phosphor layer made of cesium iodide (CsI) activated with sodium (Na) is deposited on the substrate to a thickness of about 400 .mu.m, an increase in substrate temperature caused by heat of sublimation generated when the evaporation material attaches to the incident window substrate or radiation heat generated by the evaporation apparatus is not negligible. If a phosphor layer is to be formed to a predetermined thickness within a short period of time, the substrate temperature is increased quickly, and sufficiently thin columnar crystal grains cannot be obtained. The thinner the incident window is formed to increase the X-ray transmittance, the more conspicuous the temperature increase in window substrate becomes during film formation, and sufficiently thin columnar crystals cannot be obtained. To avoid these problems, the amount of phosphor attaching to the substrate per unit time may be decreased. Then, however, a deposition time required for forming a phosphor layer to a required thickness is prolonged very much, leading to a lack in industrial practicability.
As a technique for hermetically bonding a thin aluminum X-ray incident window to a comparatively thick iron-alloy support frame, a thermocompression bonding technique in which bonding is performed by heating and pressure has been employed in practice. However, this technique merely substantially aims at bonding an X-ray incident window as part of a vacuum vessel to the main body of the vacuum vessel, and an X-ray image intensifier in which an input screen is directly formed on the inner surface of the X-ray incident window fabricated in this manner is supposed to lack in practicability. This is because deformation of the X-ray incident window due to a high pressure applied during thermocompression bonding cannot be avoided, and a high resolution cannot be obtained accordingly.
A technique in which an iron-alloy support frame and an aluminum X-ray incident window are brazed by interposing a brazing sheet between them is disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 61-253166 and Jpn. Pat. Appln. KOKOKU Publication No. 2-25704. With the brazing structure disclosed in these official gazettes, deformation of the X-ray incident window caused by bonding itself does not substantially occur. However, the bent portion where the flat portion around the X-ray incident window changes to a convex spherical surface and its inner circumferential portion close to it are not supported by a high-strength member. Thus, when this structure is completed as an X-ray image intensifier, because of the atmospheric pressure, it is found that the inner circumferential portion of the bent portion tends to be largely deformed upon application of a stress to the portion around the X-ray incident window, particularly to the bent portion. Therefore, a distortion occurs in the electron lens system, and a high resolution cannot be obtained.
In order to prevent this, a method may be possible wherein, as shown in FIG. 1, a sufficiently wide brazing sheet 23 is interposed between a flat portion 21a of an annular support frame 21 having a crank-shaped half-section and made of an iron alloy and a peripheral flat portion 22a of a convex spherical X-ray incident window 22 made of an aluminum material, and this structure is heated, thereby achieving hermetic brazing. The brazing sheet 23 consists of a core portion 23a made of an aluminum material and brazing material layers 23b and 23c integrally formed on the two surfaces of the core portion 23a as clad layers.
When brazing is performed in practice in this manner, however, the molten brazing material is fluidized to creep over from the inner surface of the flat portion 21a of the annular support frame 21 and a bent portion 22b of the X-ray incident window 22 upward to the region of the convex spherical portion 22c, and thereafter forms a solidified fluid brazing material layer B. In particular, fine corrugations are usually formed on the entire inner surface of the window to increase the adhesion strength of the CsI phosphor layer to the inner surface of the X-ray incident window. The molten brazing material during brazing tends to widely flow on the finely corrugated surface formed in this manner. Then, the fluid brazing material layer B creeps up to a region where an input screen 24 is to be formed, as shown in FIG. 2.
When the fluid brazing material layer B is present up to the prospective input screen forming region, even if the brazing material layer B is very thin, since the region of the aluminum substrate itself and the region of the brazing material layer B itself have different reflectances for a light beam emitted by the CsI phosphor layer, a luminance change boundary appears comparatively clearly particularly in the peripheral portion of an output image. Also, the adhesion strength of the phosphor layer is degraded.